Electrophotographic apparatus comprising density sensor means

Abstract

One or more marks (34), (36) are provided on a non-image area of a platen (23) which supports an original document (16). Electrostatic images of the document (16) and mark (34) are formed on a photoconductive drum (12) and developed to form toner images. The toner image of the original document (16) is transferred to a copy sheet (21) which is fed into contact with the drum (12) by a roller (119) or belt (19). The toner image of the mark (34) is transferred to the roller (119) or belt (19) and the optical density thereof sensed by a photosensor (39). The toner density, transfer charge and other parameters are controlled in accordance with the sensed optical density. An auxiliary sensor (91) may be provided to sense the optical density of the toner image of the mark (34) on the drum (12) prior to transfer for improved parameter control.

What is claimed is: 
     
       1. An electrophotographic apparatus having a platen for supporting an original document on an image area of the platen, a photoconductive member, imaging means for forming an electrostatic image of the original document on an image area of the photoconductive member, developing means for developing the electrostatic image to form a toner image, transfer means including a transfer member for supporting a copy sheet on an image area of the transfer member and moving the copy sheet into engagement with the photoconductive member for transferring the toner image to the copy sheet, characterized by comprising: mark means of predetermined optical density provided on a non-image area of the platen, the imaging means forming an electrostatic image of the mark means on a non-image area of the photoconductive member, the developing means developing the electrostatic image of the mark means to form a toner image thereof, the transfer means transferring the toner image of the mark means onto a non-image area of the transfer member;   sensor means for sensing an optical density of the toner image of the mark means on the transfer member; and   auxiliary sensor means for sensing an optical density of the toner image of the mark means on the photoconductive member before transfer to the transfer member and computing means for producing an output corresponding to a predetermined function of a difference between outputs of the sensor means and the auxiliary sensor means.   
     
     
       2. An apparatus as in claim 1, in which the sensor means comprises a photosensor. 
     
     
       3. An apparatus as in claim 1, further comprising computing means for producing an output signal when the optical density sensed by the sensor means is above a predetermined value. 
     
     
       4. An apparatus as in claim 1, further comprising computing means for producing an output signal when the optical density sensed by the sensor means is below a predetermined value. 
     
     
       5. An apparatus as in claim 1, further comprising a differential amplifier having one input connected to an output of the sensor means and another input connected to receive a reference voltage corresponding to a reference optical density. 
     
     
       6. An apparatus as in claim 5, in which an inverting input of the differential amplifier is connected to the output of the sensor means and a non-inverting input of the differential amplifier is connected to receive the reference voltage. 
     
     
       7. An apparatus as in claim 5, in which a non-inverting input of the differential amplifier is connected to the output of the sensor means and an inverting input of the differential amplifier is connected to receive the reference voltage. 
     
     
       8. An apparatus as in claim 1, in which the transfer member comprises an endless belt. 
     
     
       9. An apparatus as in claim 1, in which the transfer member comprises an electrically biased roller. 
     
     
       10. An apparatus as in claim 1, in which the photoconductive member has a rotating photoconductive surface, the sensor means being disposed downstream of the transfer member in a direction of rotation of the photoconductive surface. 
     
     
       11. An apparatus as in claim 1, in which the computing means comprises first comparator means for comparing the output of the auxiliary sensor means with a reference voltage corresponding to a reference optical density and producing an output corresponding thereto; and second comparator means for comparing the output of the first comparator means with the output of the sensor means. 
     
     
       12. An apparatus as in claim 11, in which the first comparator means comprises a voltage comparator for producing a zero output when the optical density sensed by the auxiliary sensor means is below the reference optical density and a maximum output when the optical density sensed by the auxiliary sensor means is above the reference optical density. 
     
     
       13. An apparatus as in claim 12, in which the second comparator means comprises a differential amplifier for producing an output corresponding to a difference between the outputs of the first comparator means and the sensor means. 
     
     
       14. An electrophotographic apparatus having a platen for supporting an original document on an image area of the platen, a photoconductive member, imaging means for forming an electrostatic image of the original document on an image area of the photoconductive member, developing means for developing the electrostatic image to form a toner image, transfer means including a transfer member for supporting a copy sheet on an image area of the transfer member and moving the copy sheet into engagement with the photoconductive member for transferring the toner image to the copy sheet, characterized by comprising: mark means of predetermined optical density provided on a non-image area of the platen, the imaging means forming an electrostatic image of the mark means on a non-image area of the photoconductive member, the developing means developing the electrostatic image of the mark means to form a toner image thereof, the transfer means transferring the toner image of the mark means onto a non-image area of the transfer member; and   sensor means for sensing an optical density of the toner image of the mark means on the transfer member;   the mark means comprising first and second marks of first and second different optical densities, the sensor means comprising first and second sensors for sensing optical densities of first and second toner images on the transfer member corresponding to the first and second marks respectively, the apparatus further comprising control means for controlling at least one of the imaging means, the developing means and the transfer means in accordance with a predetermined function of the sensed optical densities of the first and second toner images on the transfer member in combination.   
     
     
       15. An apparatus as in claim 14, in which the control means comprises comparator means for producing outputs indicating differences between output voltages of the first and second sensors and first and second reference voltages corresponding to first and second reference optical densities respectively.